@DTS Engineer
Thanks for the link. Developer ID Application, Apple Distribution, and Apple Development yielded a different error: "An installer signing identity (not an application signing identity) is required for signing flat-style products". 3rd Party Mac Developer Installer yielded the same error ("this identity cannot be used for signing code"). But the workaround I used works for now. Thanks again.
Topic:
Code Signing
SubTopic:
Certificates, Identifiers & Profiles